Reverse Engineering and Program Understanding

Most people interact with computer programs at a surface level, entering input and eagerly (impatiently?!) awaiting a response. The public fašade of most programs may be fairly thin, but most programs go much deeper than they appear at first glance. Programs have a preponderance of guts, where the real fun happens. These guts can be very complex. Exploiting software usually requires some level of understanding of software guts.

The single most important skill of a potential attacker is the ability to unravel the complexities of target software. This is called reverse ngineering or sometimes just reversing. Software attackers are great tool users, but exploiting software is not magic and there are no magic software exploitation tools. To break a nontrivial target program, an attacker must manipulate the target software in unusual ways. So although an attack almost always involves tools (disassemblers, scripting engines, input generators), these tools tend to be fairly basic. The real smarts remain the attacker's prerogative.

When attacking software, the basic idea is to grok the assumptions made by the people who created the system and then undermine those assumptions. (This is precisely why it is critical to identify as many assumptions as possible when designing and creating software.) Reverse engineering is an excellent approach to ferreting out assumptions, especially implicit assumptions that can be leveraged in an attack.1

Into the House of Logic

In some sense, programs wrap themselves around valuable data, making and enforcing rules about who can get to the data and when. The very edges of the program are exposed to the outside world just the way the interior of a house has doors at its public edges. Polite users go through these doors to get to the data they need that is stored inside. These are the entry points into software. The problem is that the very doors used by polite company to access software are also used by remote attackers.

Consider, for example, a very common kind of Internet-related software door, the TCP/IP port. Although there are many types of doors in a typical program, many attackers first look for TCP/IP ports. Finding TCP/IP ports is simple using a port-scanning tool. Ports provide public access to software programs, but finding the door is only the beginning. A typical program is complex, like a house made up of many rooms. The best treasure is usually found buried deep in the house. In all but the most trivial of exploits, an attacker must navigate complicated paths through public doors, journeying deep into the software house. An unfamiliar house is like a maze to an attacker. Successful navigation through this maze renders access to data and sometimes complete control over the software program itself.

Software is a set of instructions that determines what a general-purpose computer will do. Thus, in some sense, a software program is an instantiation of a particular machine (made up of the computer and its instructions). Machines like this obviously have explicit rules and well-defined behavior. Although we can watch this behavior unfold as we run a program on a machine, looking at the code and coming to an understanding of the inner workings of a program sometimes takes more effort. In some cases the source code for a program is available for us to examine; other times, it is not. Therefore, attack techniques must not always rely on having source code. In fact, some attack techniques are valuable regardless of the availability of source code. Other techniques can actually reconstruct the source code from the machine instructions. These techniques are the focus of this chapter.